1. Field of the Invention
This invention relates to a method for forming a deposition film, and in particular, to a method for forming a deposition film on a desired substrate such as photoconductive film, semiconductor film, insulating film and the like by using electric, light or thermal energy.
2. Description of the Prior Art
Heretofore, it has been known that SiH.sub.4 is decomposed by an excitation energy such as discharge, thermal energy and the like to form a deposition film of amorphous silicon (hereinafter referred to as "a-Si") on a substrate and the resulting deposition film is used for various purposes.
However, conventional methods for forming a-Si deposition films using SiH.sub.4 as the starting material have the following drawbacks to be solved:
(1) The film forming velocity is slow when a high quality of the film is to be kept; PA1 (2) High excitation energy is required regardless of the type of the fed energy; and PA1 (3) Upon forming a large area or thick deposition film, it is difficult to ensure uniform electric or optical characteristics and stable quality, and there are liable to be caused inrregularity of the film surface and defects in the bulk of the film during the formation of the film.
For the purpose of solving such problems, a method for forming an a-Si deposition film using Si.sub.2 H.sub.6 as a starting material has been recently proposed and come into the limelight, but is still insufficient to solve the problems. In particular, when heat energy is used as the excitation energy source, a high temperature of at least 400.degree. C. is necessary and therefore, lowering the necessary energy and making the thermal efficiency higher are still to be solved in future. Since Si.sub.2 H.sub.6 is decomposed to easily form SiH.sub.4 and excitation decoposition products, Si.sub.2 H.sub.6 is better than SiH.sub.4. However, from the standpoint of reaction theory, efficient decomposition of Si.sub.2 H.sub.6 can be conducted when Si.sub.4, a main decomposition product of the decomposition, successively decomposes further. Therefore, the remarkably efficient decomposition can be expected only limitedly.
In addition, Si.sub.2 H.sub.6 is not a material easily available and its production process has not yet been eastablished so that Si.sub.2 H.sub.6 is expensive. Thus, the method using Si.sub.2 H.sub.6 can not be practically carried out from the economical point of view. U.S. Pat. No. 4,363,828 discloses that higher silanes such as Si.sub.2 H.sub.6 and the higher ones can be used. However, there is not any actual example that such higher silanes are produced at a high yield and the deposition films are practically utilized as a useful material. Furthermore, the commercial processes will be completed only after practical processes for producing the higher silanes and techniques for utilization thereof have been developed.
If the fabrication of the deposition film can be effected at a low energy level, the formation of the deposition film having uniformity and high quality can be expected and the fabrication conditions can be easily controlled, and moreover, the commercial productivity including reproducibility can be improved. For example, in the case of glow discharge deposition, the effect of discharge energy of high power on the deposition film during the formation thereof is so large that it is difficult at present to control stably the fabrication conditions ensuring reproducibility.
In the case of a thermal energy deposition process, a high temperature is required so that the substrate material for forming the deposition film is limited, and in addition, the probability of releasing of the useful bonded hydrogen atoms in a-Si increases and thereby the desired characteristics are obtained with difficulty. Furthermore, the decomposition efficiency is so low that the deposition velocity is slow and, therefore, the process is not suitable for mass production.
On the other hand, heretofore, it has been also proposed to use silane gases of a higher order such as ##STR1## and the like, in addition to the above-mentioned silane gases such as SiH.sub.4, Si.sub.2 H.sub.6 and the like. However, SiH.sub.4, S.sub.2 H.sub.6 and a series of silanes as mentioned above are explosively burned with oxygen in air so that the handling is very difficult.